OBJECTIVES: (1) In order to determine the binding of the two myosin heads we will measure HMM induced actin polymerization. Second, HMM and S-1 will be compared with respect to the number of heads required to cause potentiation. (2) S-1 with actin and ATP sites uncoupled from each other will be used to determine the number of myosin heads needed for maximal potentiation. The effect of potentiation on dissociation of acto S-1 AMPNP also will provide such information. (3) The role of tropomyosin and troponin T in the steepness of the calcium switch will be determined.